generator motoring 2

[dortheo]

A stream turbine/generator (70mw) trips and the main steam valve closes, the generator breaker is still closed and is tied to the grid. 15 mins later the operator notices the turbine/gen is still turning at 3600rpm.( with no power output). The 32 relay was checked under trip conditions 3 months previously and worked flawlessly as it has in the past.The relay was not picked up during this incident. Why was the turbine/gen still running at speed after 15mins? When the gen. breaker was opened manually the speed dropped off to zero.Was this equipment motoring?

 

[dpc]

It sounds like it was motoring. It doesn't take a lot of power to spin the turb-gen under no-load conditions. This gives the 32 relay a tough job to do since it is operating off CTs that are sized for the full output of the generator. It may just be that the settings of the 32 relay were not low enough to detect the trip.

How did you test the relay? If bench-tested, that still leaves open the possibility that the CTs or PTs are mis-wired.

 

[ScottyUK]

Hello Dortheo,

The reverse power relay acts as a wattmeter measuring positive power under generating conditions. If the unit senses a negative power signal i.e. machine motoring then a trip signal is initiated. However, the relay uses the instrument current and voltage transformers common to all other protection relays.

The actual power required to motor the generator and turbine is a small fraction of the rated output power of the generator (approximately 3%). In order to detect this small level of power the voltage and current transformers must be very accurate.

Under normal conditions when the generator is de-loaded to be removed from service the current reduces but the voltage level is maintained. The relay is optimised to operate over a small range of current and power. Therefore as the machine is de-loaded the current and power reduce together and the errors introduced by the instrumentation are small. Thus the relay operates reliably.

However, certain conditions with high reactive loads can produce phase errors in the instrumentation. Under these conditions the relay appears to be measuring positive power even though the unit is in a motoring condition. Therefore the relay will not operate.

Do you know what the reactive load on your generator was at the time of trip?

 

[jbartos]

Suggestion: Implement the generator breaker opening function from the turbine valve trip control as a backup to the relay 32.

 

[ScottyUK]

jB,

When the unit is tripped or removed from service the governor valves are closed thus removing input energy to the turbine. However, entrained steam still remains within the pipework and turbine casing. Therefore, the main generator breaker is left closed to allow this stored energy to be dissipated into the electrical network under a controlled condition. If this were not the case then there would be danger of over-speeding the machine.

 

[alehman]

Do you depend solely on the 32 to trip the generator breaker? Usually 32 is a backup function. Can you trip the generator breaker after a time delay from turbine trip?

 

[RJF]

The turbine / generator manufaturer should be able to give you an idea of what amount of power the TG will pull under a reverse power (no steam) situation. On our newer machines we use a dual system that combines a contact from the stop valve with a timer and a reverse power relay with a timer. We had a few instances where a system that checked out ok under test conditions (intentional reverse power with low kvar out of machine) did not trip correctly when steam was lost (either though a boiler problem or a closed rack - stop valve was not closed). We found in these cases that the Generator was still putting out normal KVAR but was running at negative KW of about 3%. The 32 relay in this case sees a very low PF. Turns out that the static 32R relay did not work correctly at less than .1 pf. Took a while for the manufacturer to admit that. We actually upgraded the relaying to include a Beckwith multifuntion digital relay (3425)- it was sensitive enough and was did not have a problem at low power factor. The SEL generator relay was also tested and worked correctly. Another manufacturer's digital relay was tested and would not work correctly at low PF.

 

[xabproject]

Dear Dortheo,
The simplest / easiest way we found was to automatically drive the AVR pot to zero on 'turbine trip+ESV closed' condition instaed of changing the Rev Pow relay. This was because during one such incident an operator reduced the excitation and the relay did pick-up and it was later on conclusively prooved that the heavy VAR loading ('heavy/large'compared to watt component at that time)does affect the CT/Relay combined performance.
Good luck and best regards